Solid alkali metal halides of the formula MX, where M is an alkali metal and X is a halide have a moderate ionic conductivity, and a low electronic conductivity. Conductivity takes place through M+X vacancy mechanism. An order of magnitude of conductivity can be gained (10.sup.-5 mho cm.sup.-1 at ambient temperature) by the mixing of LiI and Al.sub.2 O.sub.3 powders and pressing into pellet form. Possible explanations are: (a) interfacial Li.sup.+ conduction; (b) the increase of Li.sup.+ vacancies concentration due to the intimate presence of Al.sup.3+ at the LiI interface, the Li.sup.+ conduction being mainly at the LiI/Al.sub.2 O.sub.3 interface. On this basis an all solid Li/LiI-Al.sub.2 O.sub.3 /PbI.sub.2 primary battery was developed.
However the LiI-Al.sub.2 O.sub.3 pellets are very brittle and have little mechanical strength and are sensitive to thermal shock.
Attempts to use Al.sub.2 O.sub.3 with polymers to improve the mechanical strength, in conjunction with lithium salts resulted in a too low Li transference number as too low salt concentrations or too large Al.sub.2 O.sub.3 particles or unsuitable salts were used.
Known polymer electrolytes (PEs) have a number of drawbacks, such as a lithium transference number of only about 0.3 to 0.5 which leads to high concentration polarization and interface problems. Frequently salt anions such as BF.sub.4.sup.-, AsF.sub.6.sup.- and ClO.sub.4.sup.- were used which are not compatible with lithium. The above results in a high interfacial Li/PE resistance and lithium passivation. Furthermore the mechanical strength is not adequate, especially at elevated temperatures. The drawbacks set out above deterred the development of high-power high energy alkali metal (M) batteries using polymer electrolytes. Reactivity of the prior art anions with lithium or other alkali metals results in a thick passivating layer, having a high resistance. Furthermore polymer electrolytes tend to soften and start to flow at temperatures in the 100.degree. C. range.
Recently, Nagasubramanian, Peled, Attia and Halpert, ECS Meeting, Toronto, Canada, October 1992, fabricated CSE films made of 12-17% 0.05.mu. Al.sub.2 O.sub.3 particles, 40-55% LiI and 35-53% Polyethylene Oxide (PEO). At 100.degree. C. the transference number of lithium cation in these films is 0.8-1.0, depending on the film composition.
The state of the art Li/PE batteries which use cathodes such as V.sub.2 O.sub.5, V.sub.3 O.sub.13, Li.sub.0.5 M.sub.n O.sub.2 do not have an internal electrochemical overcharge protection mechanism. Thus it is not practical to stack these cells in a series combination in a bipolar configuration.